The present invention relates to the manufacture of a magnetic recording medium having a recording layer made of a ferromagnetic metal layer and, more particularly, to a method of and an apparatus for making the magnetic recording tape free from wrinkles.
A magnetic recording medium, for example, a magnetic recording tape, having a ferromagnetic metallic recording layer is currently manufactured by feeding a base of, for example, polyester or the like from a supply reel towards a take-up reel within a vacuum chamber around a cylindrical drum, and vapor depositing a ferromagnetic material on the base to form the recording layer thereby completing the length of magnetic recording tape. For avoiding the occurrence of wrinkles on the tape, the prior art technique is to adjust the drive torque produced by each of the supply and take-up reels thereby to adjust the tension of the tape travelling from one guide roll to another during the passage thereof from the supply reel onto the take-up reel.
In addition, in order to improve an electrical characteristic of the tape, the one having its surface rendered as smooth as possible is conventionally used. On the other hand, in order to improve thermal contact between the tape and the cylindrical drum during the vacuum vapor depositing so that the temperature of the tape can be favorably controlled, the drum, the outer peripheral surface of which has a ten-point average surface roughness of smaller than 1.0 .mu.m, is generally utilized.
According to the conventional method wherein only the drive torques of the supply and take-up reels are adjusted to adjust the tension of the tape, in order to avoid thermal damage to the tape and any possible fluctuating run of the tape during the vacuum vapor deposition, a relatively high tension must be imparted to the tape to make a good thermal contact between a portion of the tape and the drum. The application of such a high tension has been found problematic in that not only is the tape wound too tightly on the take-up reel, but also the tape is susceptible to the occurrence of wrinkles.
Also, the use of the drum, having a smooth outer peripheral surface, in combination with the tape having a smooth surface has been found to be a problem. Specifically, in view of the tape being electrostatically charged with reflected and secondary electrons, caused by an electronic gun used to evaporate the ferromagnetic material, and/or by the effect of friction between the tape and any one of the guide rolls, the tape tends to stick to the cylindrical drum as well as the guide rolls to such an extent as to result in the fluctuating run of the tape, thereby rendering the tape susceptible to the occurrence of wrinkles. In particular, since the opposite side edge portions of the tape are not only insufficiently tensioned, but also are easy to be electrostatically charged with the result that a smooth relative slip may not occur between the tape and the cylindrical drum, the tape tends to be wound onto the take-up reel in the form wherein opposite side edge portions thereof are curled or buckled, resulting in the occurrence of wrinkles.
Furthermore, the use of a drum of which the outer peripheral surface has a ten-point average surface roughness smaller than 0.1 .mu.m as measured according to Japan Industrial Standard (JIS) B 0601 has the following problem.
As is well known to those skilled in the art, the surface of the length of magnetic recording tape opposite to the recording layer formed by depositing the ferromagnetic material is very smooth. On the other hand, the outer peripheral surface of the cylindrical drum hitherto used is also very smooth and is normally cooled to about -20.degree. C. during the manufacture of the magnetic recording tape. Therefore, considering that, during the deposit of the ferromagnetic material within the vacuum chamber, the base is cooled in contract with the drum and, on the other hand, heated in contact with vapor of the ferromagnetic material which is usually about 2,000.degree. C. in temperature. With the base so exposed to the severe conditions within the vacuum chamber, the tape undergoes cyclic contraction and elongation during its passage around the drum and, therefore, a smooth slip of the tape relative to the drum is often hampered. This in turn constitutes a cause of strain in the tape leading to the occurrence of wrinkles on the length of magnetic recording tape.